#include <bits/stdc++.h>
using namespace std;

int CautBin(int n)
{
    int st, dr, mid, rasp;
    st = 1; dr = n;
    while (st <= dr)
    {
        mid = (st + dr) / 2;
        cout << "? " << mid << '\n';
        cout.flush();
        cin >> rasp;

        if (rasp == 0) return mid;
        if (rasp < 0) st = mid + 1;
        else dr = mid - 1;
    }
    return -1;
}

int main()
{
    int n, sol;
    cin >> n;
    sol = CautBin(n);
    cout << "! " << sol << '\n';
    return 0;
}